The instant invention relates to the construction of fiber reinforced plastic structures, and more particularly to a method of forming a hollow structure, such as a hull of a sailboard, from a fiber reinforced plastic material in a female mold.
Fiber reinforced plastic structures are generally formed by applying uncured fiber reinforced plastic materials to either the exterior surfaces of male molds, which are formed in the configurations of desired structures, or to the interior surfaces of female molds which are formed in configurations that are complimentary to those of desired structures. In this regard, it has been found that male molds are generally easier to construct than female molds, because they are formed in the configurations of desired structures rather than in complimentary configurations. Still further, it has been found that it is possible to form seamless fiber reinforced plastic shells over some male molds. Filament-wound fuel tanks formed over mandrels, and sailboard hulls formed over shaped foam blanks are examples of structures made by this technique. Unfortunately, however, since the outer surfaces of structures formed around male molds are not formed against finished mold surfaces they generally require substantial amounts of finish work. Further, when seamless hollow structures are formed around mandrels, the mandrels are generally not removable from within the structures thereof without being destroyed.
Because structures which are formed on male molds generally require substantial amounts of outer surface finish work, it has been found that it is generally preferable to form composite structure from fiber reinforced plastic material in female molds whenever several structures are to be built from one mold, and the structures must have finished outer surfaces. In this regard, since the outer surfaces of components formed in female molds are generally formed against smooth mold surfaces they generally require relatively little finish work after they have been removed from the molds thereof. Further, it has been found that female molds can generally be re-used, even when they are used to form hollow structures. However, because of the inaccessibility of the interiors of female molds which define substantially enclosed interior areas, heretofore it has frequently not been practical to form seamless hollow structures in female molds. Consequently, heretofore it has generally been necessary to form hollow structures by forming two or more structure sections in separate female molds and then gluing or otherwise bonding the sections together. The disadvantage of such a construction technique is that in cases where the interiors of hollow structures are inaccessible after the sections thereof have been glued or otherwise bonded together it is impossible to apply fiber reinforcement to the inner sides of the seam areas where the two or more sections are glued or bonded together. Further, if the exterior of the finished structure is reinforced in the seam areas substantial amounts of finish work are generally required to finish these outer surface areas.
While heretofore it has been known to form seamless hollow structures utilizing multi-piece female molds which define substantially enclosed interior areas, the heretofore known techniques have generally been limited to applications requiring low pressure during curing. Specifically, the heretofore known techniques have comprised applying uncured fiber reinforced plastic materials to the inner surfaces of mold sections, and then assembling the mold sections with the uncured fiber reinforced materials therein around inflatable bladders which are used to apply outward forces to the fiber reinforced plastic materials during curing. However, it has been found that because the cumulative outward forces applied to female molds by inflatable bladders are extremely high, even when the bladders are inflated with relatively low pressure air or gas, it is generally necessary to utilize extremely strong, highly reinforced molds to effectively apply such techniques. Hence, heretofore it has generally not been practical to form fiber reinforced plastic structures in female molds utilizing inflatable bladders when constructing multi-layered laminate structures, such as honeycomb-cored structures, which require significant levels of outward pressure (generally at least 13 psi) to maintain the laminations in proper orientation, and in pressurized contact prior to and during curing. For these reasons, the processes which have been heretofore available for manufacturing fiber reinforced plastic structures that require high pressure during curing and finished outer surfaces have generally been unsatisfactory for production operations.
The instant invention provides an effective method of forming hollow structures, as well as an effective method of forming interior joints in at least partially enclosed confined interior areas of structures. Specifically, the method of the instant invention as it is applied to forming a hollow structure of a predetermined configuration, comprises the steps of assembling a composite layer assembly and a vacuum bag in a female mold having an inner configuration which is complimentary to the desired predetermined outer configuration of the hollow structure. The composite layer assembly comprises, at least in part, an uncured fiber reinforced plastic material, and it is assembled in the mold so that it substantially covers the inner surface of the mold. The vacuum bag is assembled so that it is received in the interior area defined by the composite layer assembly, so that the vacuum bag and the composite layer assembly cooperate to define an evacuation layer or area therebetween. In the preferred method, a release layer and a vacuum "breather" layer are assembled in the mold between the composite layer assembly and the vacuum bag, so that the release layer is adhered to the composite layer by the inherent tackiness of the uncured composite layer assembly, and so that the breather layer can communicate vacuum to substantially the entire outer surface of the vacuum bag. The vacuum bag is then adhesively secured to the release layer at various locations in order to effectively retain the vacuum bag in position along substantially the entire inner surface of the composite layer assembly. In this regard, when adhesively securing the vacuum bag to the release layer particular attention is preferably given to securing it to the release layer so that the vacuum bag extends into any inside corners, cavities, or other areas of irregular configuration, to enable the vacuum bag to be effectively utilized for applying outward pressure to the composite layer assembly in these areas. In any event, once the composite layer assembly, the vacuum bag, the breather layer and the release layer have been installed in the interior of the female mold and the mold has been fully assembled in a closed position, the method is further carried out by venting the interior of the vacuum bag to the atmosphere, and evacuating the evacuation area between the vacuum bag and the composite layer assembly so that the air in the interior of the vacuum bag operates to apply outward pressure of normally between approximately 13 and 14 psi to the inner surface of the composite layer assembly. Thereafter, the method is further carried out by curing the composite layer assembly to form a cured composite layer assembly, releasing the vacuum on the evacuation area, and removing the cured composite layer assembly from the mold.
The composite layer assembly preferably comprises fiber reinforced plastic inner and outer layers and a core layer therebetween for maintaining the inner and outer fiber reinforced plastic layers in spaced relation. Further, the composite layer assembly is preferably formed in at least two composite layer assembly sections which correspond to sections of the female mold. The at least two composite layer assembly sections are assembled in the mold sections thereof, and a fiber reinforced plastic overlap layer is assembled therewith so that it overlaps mating composite layer assembly sections when they are received in assembled relation to create a structure with continuous fiber reinforcement rather than glued seams. Still further, an uncured foaming epoxy core splice material is preferably assembled in the corner areas between adjacent composite layer assembly sections to fill any voids in the corner areas of the composite layer assembly.
The method of forming a hollow structure of the subject invention preferably further includes a step of removing the vacuum bag, the breather layer, and the release layer from the cured composite layer assembly after removing the cured composite layer assembly from the mold. This step is preferably carried out by forming an access hole in the cured composite layer assembly, drawing adjacent portions of the vacuum bag, the breather layer, and the release layer outwardly through the access opening and twisting the adjacent sections of the vacuum bag, the breather layer, and the release layer relative to the cured composite layer assembly in order to separate the vacuum bag, the breather layer, and the release layer therefrom. Thereafter, the vacuum bag, the breather layer, and the release layer are pulled outwardly through the access hole.
The method of the subject invention can also be utilized for forming a hollow member having an inner support wall, or like structure therein. In this case, an inner support wall or the like comprising a pair of spaced fiber reinforced plastic layers and an inner core layer therebetween is assembled in the interior of a composite layer assembly so that the support wall extends between spaced portions of the composite layer assembly. Uncured reinforcing fiber reinforced plastic layers are assembled in the joint areas, and uncured foaming epoxy or core splice strips are preferably also assembled in the joint areas. The vacuum bag is adhesively secured in position in the joint areas between the support wall and the composite layer assembly, so that the vacuum bag is operative for effectively applying outward pressure to the laminations in the joint areas between the support wall and the composite layer assembly in order to retain these laminations and the support wall in position during the curing process.
The subject invention can also be effectively utilized for forming a reinforced interior joint at an intersection between adjacent elements in an at least partially enclosed area in a hollow member that would otherwise be inaccessible and require gluing. In this case, the method is carried out by assembling an uncured fiber reinforced plastic material in the particular joint area so that it extends between the adjacent elements at the intersection, and then assembling a vacuum bag, and preferably also a release layer and a breather layer, in the at least partially enclosed area so that the vacuum bag extends along the uncured fiber reinforced plastic materials and cooperates therewith to define an evacuation area. The evacuation area is then evacuated so that the vacuum bag, and preferably also the release layer and the breather layer, are drawn toward the fiber reinforced plastic materials at the intersection to retain the fiber reinforced plastic materials in position during the curing operation.
It has been found that the method of the instant invention can be effectively utilized for forming hollow structures. Specifically, it has been found that by assembling a vacuum bag in the interior of an uncured composite layer assembly of a hollow structure in a female mold and evacuating the area between the vacuum bag and the composite layer assembly, the composite layer assembly can be effectively held in position during a curing operation in order to eliminate voids in the cured composite layer assembly. It has been further found that by adhesively securing a vacuum bag in position adjacent any inner corners or the like in a composite layer assembly, the vacuum bag can be more effectively held in position for applying outward pressure to the composite layer assembly during a curing process. It has been even still further found that by assembling an uncured foaming epoxy material in the joint areas between sections of a composite layer assembly and applying overlapping fiber reinforced plastic layers to these joint areas, the final finished composite layer assembly formed by the method of the subject invention can be made so that it is essentially seamless in appearance and is substantially free from voids.
Accordingly, it is a primary object of the invention to provide a method which combines the advantages of male and female molds in the construction of seamless fiber reinforced plastic hollow structures.
Another object of the instant invention is to provide a process for forming fiber reinforced joints at confined, enclosed, or otherwise inaccessible intersections between composite parts.
Another object of the instant invention is to provide an effective method of forming a hollow structure from a composite layer assembly comprising an uncured fiber reinforced plastic material.
Still another object of the instant invention is to provide an effective method of forming a hollow structure by assembling an uncured composite layer assembly and a vacuum bag in a female mold, venting the interior of the vacuum bag to atmosphere and evacuating the area between the vacuum bag and the composite layer assembly in order to apply outward pressure thereto.
An even still further object of the instant invention is to provide a method of forming an interior seam at an intersection between adjacent elements in an at least partially enclosed area in a hollow structure.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.